It has been suggested that a computer is a thermodynamic engine that sucks entropy out of data, turns that entropy into heat, and dumps the heat into the environment. The ability of prior art thermal management technology to get that waste heat out of semiconductor circuits and into the environment, at a reasonable cost, has limited the density and clock speeds of electronic systems.
A typical characteristic of heat transfer devices for electronic systems is that the atmosphere is the final heat sink. In heat transfer devices for electronics today, a semiconductor chip is often arranged so as to thermally contact a passive heat spreader plate, which conducts the heat from the chip to one of several types of fins. The fins, in turn, convect heat to the atmosphere with natural or forced air convection. As the power to be dissipated by semiconductor devices increases with time, a problem arises in that the thermal conductivity of the available materials becomes too low to conduct the heat from the semiconductor devices to fins with an acceptable temperature drop. The thermal power density emerging from semiconductor devices today is so high that even solid copper or silver spreader plates are not adequate.
Thermal energy can sometimes be transported by an intermediate loop of recirculating fluid. Heat from a hot object is conducted into a heat transfer fluid, the fluid is pumped by some means to a different location, where the heat is conducted out of the fluid into a convective fin and finally into the atmosphere. For example, U.S. Pat. Nos. 5,125,451 and 5,274,920, issued to Matthews, provide a microscopic laminar-flow heat exchanger for cooling a heat generating device, such as a semiconductor integrated circuit. The heat exchanger consists of a plurality of thin plates which have been laminated together to form a block. The plates consist of thin copper foil strips each having a microscopic recessed portion etched into one face of the plate. These recessed portions are chemically etched to a shallow dimension, on the order of fifty microns deep, prior to lamination. Either before or after the plates are laminated together, holes are cut through the plates at opposite sides of the recessed portions such that when the stack is laminated the holes align to form a pair of coolant distribution manifolds. Each of the manifolds is essentially a tube which penetrates into the block. The tubes are connected via the plurality of microscopic channels formed from the recessed portions during the lamination process. Selectively adjusting the thickness of the channels and the coolant flow rate allows the block to function as a heat exchanger. A semiconductor die is simply placed or bonded onto the surface of the block to effectuate heat removal.
A significant disadvantage with structures such as are taught by Matthews is the limited surface area available for coolant contact and conductive heat transfer. Additional available surface area or a more serpentine coolant flow path, if provided, would greatly enhance the heat transfer characteristics of such devices. Unfortunately, the microscopic size of Matthew's devices, and the etching techniques used to manufacture such devices, do not provide for any meaningful increase in internal surface area or complex coolant flow paths, thereby limiting the amount of thermal energy that can be removed by a single device. Furthermore, such structures do not lend themselves easily to the use of internal structures for the creation of turbulence in the coolant as it flows through the device.
Although the creation of turbulence in coolant as it flows through a thermal transfer device is a well known technique for improving heat transfer, others have found that improved thermal performance can be achieved by configuring a fluid cooling device to support laminar fluid flow. For example, in U.S. Pat. No. 6,634,421, issued to Ognibene et al., a fluid cooling device is disclosed that includes a plurality of cold plate members, each having a plurality of imperforate plate portions and perforate portions arranged in a line with at least one connector for connecting the plate portions together at one end. The cold plate members are arranged in a stack, with respective plate portions of each cold plate member being in registration with perforate portions formed in its adjacent cold plate members in the stack. The fluid cooling device appears to provide heat transfer by close clearance laminar developing flow, which may increase the thermal performance of the fluid cooling device while maintaining low pressure drop.
None of the prior art has proved to be universally appropriate for achieving efficient thermal transfer in electronics systems.